Dynamics of ballistic photocurrents driven by Coulomb scattering

Abstract

First principles real-time time dependent density functional theory (rt-TDDFT) calculations reveal the existence of ballistic photocurrents generated by Coulomb scattering, which has not previously been considered as a mechanism for the bulk photovoltaic effect. With monolayer GeS as an example, it is predicted that ballistic currents can be comparable to shift currents under experimentally accessible conditions.

Figures (3)

• Figure 1: a) Ballistic photocurrent time-dependence of monolayer GeS under monochromatic continuous wave excitation with varying frequency and electric field strength 0.1 V/nm, turned on at $t = 0$. Dashed black lines indicate linear fit at early times. b) Total photocurrent under the same excitation conditions, displayed as a time-integral to illustrate changes in the direct current (dc) component. A linear rise in the time-integrated current indicates a constant current state.
• Figure 2: Band structure of monolayer GeS with instantaneous symmetric carrier occupation number, antisymmetric carrier occupation number, and ballistic photocurrent superimposed, for $t = 10~\mathrm{fs}$ after the start of continuous wave photoexcitation (a-c), and $t = 30~\mathrm{fs}$ after the start of continuous wave photoexcitation (d-f). Monochromatic excitation is at frequency 3.2 eV and electric field strength 0.1 V/nm. Inset in (d) denotes the path $\Gamma$ - $Z$ - $T$ - $Y$ along which the band structure is plotted. Insets i, ii in (b,e) denotes two regions of carrier accumulation displaying different dynamics.
• Figure 3: Rate of change of ballistic photocurrent at early times showing scaling with light intensity, for varying excitation energies. a) Data collected across the entire intensity range shows the onset of strong field behavior at around $E\sim 1$V/nm. b) Low field data. Colored lines are linear fits of data points, dashed lines are quadratic fits of data (including linear terms) .